Heat exchangers in air-conditioning equipment installed in buildings and automobiles generally contain fins which are fabricated in a specified shape from aluminum material (hereinafter signifying aluminum or its alloys) and the like. Such fins are arranged at closer intervals for the purpose of raising the heat exchange efficiency or under the influence of size reduction of air-conditioning equipment and, as a consequence, water condenses into globules on the surface of fins and forms a bridge between fins during the cooling operation. This phenomenon produces such undesirable results as increased resistance to air flow and lowered heat exchange efficiency.
A remedial practice has been a surface treatment to give hydrophilicity to the surface of fins as effected by applying hydrophilic coatings to fin materials prior to their fabrication into fins. The aim of this treatment is letting the enhanced hydrophilicity help condensing water to spread over the whole surface of the fins without forming globules and thus preventing the generation of the aforementioned phenomenon of bridging. Fin materials treated prior to fabrication with hydrophilic coatings as mentioned above are usually referred to as precoated fin materials.
Hydrophilic coatings of this kind, for example, contain silica and organic polymers as main components Japan Kokai Tokkyo Koho No. Sho 53-92846 (1978), Japan Kokai Tokkyo Koho No. Sho 55-99976 (1980), and Japan Kokai Tokkyo Koho No. Hei 6-221786 (1994)!. Other examples are hydrophilic coatings containing surfactants in order to enhance initial hydrophilicity Japan Kokai Tokkyo Koho No. Hei 7-188585 (1995)! and hydrophilic coatings obtained by mixing the aforementioned main components with silanes and dialkylsulfosuccinate ester salts and allowing the mixture to react at 50.degree. C. or above Japan Tokkyo Koho No. Hei 1-36503 (1989) and Japan Kokai Tokkyo Koho No. Hei 1-223188 (1989)!.
The conventional hydrophilic coatings mainly composed of silica and organic polymers exhibit good hydrophilicity in the initial stage, but they tend to lose hydrophilicity gradually with the passage of time as the result of exposure to repeated dry and moist condition and deposition of air-borne hydrocarbons. Hence, there is a demand for sustained hydrophilicity.
Judging from their compositions, the hydrophilic coatings proposed in the aforementioned patent specifications yield an alkaline bath in use and, as described later, they tend to form a relatively smooth surface when applied to fin materials. Fins with such smooth surface not only lack sufficient ability to maintain hydrophilicity, but also generate another phenomenon of repelling water. In Example 1 of the aforementioned Japan Tokkyo Kokai Koho No. Hei 6-221786 (1994), a mixture (alkaline) of a silicate (sodium silicate), water-soluble resins poly(sodium acrylate)! and water is applied to an aluminum plate to provide a fin material precoated with a hydrophilic film containing silica particles averaging 0.5 .mu.m in diameter. The alkaline nature of the film thus formed, however, presents such problems as foaming and degradation of volatile press oil applied during the fabrication of fins, adverse effects on the corrosion resistance of fin materials, and changes in the film structure caused by elution of alkali with the passage of time.
It is described in Japan Kokai Tokkyo Koho No. Hei 7-188585 (1995) that there is no restriction on the kind of surfactants to be used, that the role of the surfactants is mainly to improve the initial hydrophilicity and prevent lowering of hydrophilicity by adsorption of press oil to be applied prior to fabrication, and that addition of crosslinking agents in a suitable amount is effective for securing sustained hydrophilicity over a prolonged period of time. The hydrophilic coatings described in the aforementioned patent specification, however, were not satisfactory to provide sustained hydrophilicity, particularly after application of volatile press oil. Furthermore, it is described in Japan Tokkyo Koho No. Hei 1-36503 (1989) and others that surfactants (dialkylsulfosuccinate ester salts) are heated at 50.degree. C. or above to effect crosslinking with polymers in order to prevent lowering of the hydrophilicity due to running-off of surfactants during oil removal or in practical use. In spite of such heat treatment, the hydrophilic coatings lacked the ability to provide sustained hydrophilicity after application of volatile press oil. Moreover, the hydrophilicity tends to degrade markedly when the heat treatment after the application of the coatings is effected at high temperatures (approximately 230.degree. C. or above) or the coatings tend to yield films of uneven appearance with varying hydrophilicity when the content of surfactants added to the coatings exceeds 0.5% by weight.
Precoated fin materials are normally rolled up for storage prior to fabrication and, when treated with the conventional coatings, they sometimes undergo blocking, that is, they stick together and do not separate.
An object of this invention is to solve the aforementioned problems of prior art and to provide water-based hydrophilic coatings which can be applied, for example, to the surface of aluminum materials not only with excellent hydrophilicity in the initial stage but also with sustained hydrophilicity after pre-fabrication application of volatile press oil or after high-temperature heat treatment.
Another object of this invention is to provide a process for manufacturing precoated fin materials of high quality for heat exchangers, said fin materials being excellently hydrophilic and blocking-free when rolled up.